Generator systems that are installed in aircraft may include three separate brushless generators, namely, a permanent magnet generator (PMG), an exciter, and a main generator. The PMG includes permanent magnets on its rotor. When the PMG rotates, AC currents are induced in stator windings of the PMG. These AC currents are typically fed to a regulator or a control device, which in turn outputs a DC current. This DC current next is provided to stator windings of the exciter. As the rotor of the exciter rotates, three phases of AC current are typically induced in the rotor windings. Rectifier circuits that rotate with the rotor of the exciter rectify this three-phase AC current, and the resulting DC currents are provided to the rotor windings of the main generator. Finally, as the rotor of the main generator rotates, three phases of AC current are typically induced in its stator windings, and this three-phase AC output can then be provided to a load such as, for example, electrical aircraft systems.
Because the generators installed in aircraft will often be variable frequency generators that rotate in the speed range of 12,000 rpm to 24,000 rpm, large centrifugal forces are imposed upon the rotors of the generators. Given these stressful operating conditions, the rotors of the generators must be carefully designed and manufactured, both so that the rotors are reliable and also so that the rotors are precisely balanced. Improper balancing in particular not only can result in inefficiencies in the operation of the generators, but also potentially risk failures in the generators.
Among the important components in rotors that must be carefully designed and manufactured in order to guarantee reliability and proper balancing of the rotors are the wire coils of the rotors. The centrifugal forces experienced by the rotors are sufficiently strong as to cause bending of the wires of these coils, which over time can result in mechanical breakdown of the wires. Additionally, because the coils are assemblies of individual wires that can move to some extent with respect to one another and with respect to the remaining portions of the rotors, the coils constitute one of the significant potential sources of imbalance within the rotors. Even asymmetrical movements of these coils on the order of only a few thousandths of an inch can be significant.
In order to improve the strength and reliability of the wire coils and to minimize the amount of imbalance in the rotors that occurs due to the wire coils, wedges may be inserted in between neighboring poles of the rotors. The wedges in particular serve as physical barriers beyond which the wires of the coils cannot bend or move, and in many embodiments provide some pressure onto the coils that helps to maintain the physical arrangement of the coils.
Although the wedges employed in conventional rotors are capable of providing these benefits to some extent, the design of these conventional rotors and wedges limits the wedges' effectiveness. Just as the wires of the coils of a rotor experience high centrifugal forces as the rotor rotates at high speeds, the wedges also experience high centrifugal forces. These forces tend to cause the wedges to spread radially outward away from the shaft of the rotor during operation, thus limiting the wedges ability to confine and place pressure upon the wire coils. Particularly, insofar as the axial lengths of conventional rotors are often relatively large in comparison with the diameters of the rotors, the centrifugal forces often tend to cause significant radial deflection or flexure of the wedges near their axial midpoints.
In order to prevent the wedges from spreading radially outward, many conventional rotors employ bands around the circumferences of the rotors to retain the wedges. In other conventional rotors, an “underwedge” system is employed in which the wedges extend in their arc length all of the way between neighboring pole tips on the rotors, and snap rings are then employed to hold the wedges in place relative to the poles.
Yet these conventional structures for retaining wedges in place on rotors are limited in their effectiveness. Both the bands used to retain the wedges and the components of the underwedge systems (particularly the snap rings) also can suffer from bending during operation of the rotors. Because these devices suffer bending, the devices can only provide a limited amount of counteracting force to keep the wedges in place, and further can create additional imbalance in the rotors. Additionally, because it is difficult to accurately control the positioning of, and the amount of pressure applied by, the bands and underwedge componentry, it is difficult to accurately set and maintain the positioning of the wedges and to control the concentricity of the various wedges around the rotors.
Hence, there is a need for a new system and method for retaining wedges in a rotor. In particular, there is a need for a new system and method that allows for sufficient radial retention of the wedges of the rotor even at high speeds of operation, so that the wedges continue to provide support for and direct pressure towards the wire coils throughout operation of the generator. Further, it would be advantageous if the new system and method did not require components that had a tendency to bend in such a way as to create imbalance in the rotor. It would additionally be advantageous if the system and method allowed for the accurate positioning of wedges onto the rotor so as to provide concentricity of the rotor and its wedges. It would further be advantageous if the system was designed so as to allow the wedges to conduct heat away from the coils. It would additionally be advantageous if the system and method were relatively simple and inexpensive to implement.